For heating a surface of an object, there has been proposed a method in which bar-type electric heaters 25 are fixed in proximity to the object surface as shown in FIG. 9. In this method, however, the heating sources are separated so that the surface temperature will be non-uniform. Further, the bar-type electric heaters 25, which are the heating sources, are apart from the surface of a base plate 21, so that the temperature response on the surface is delayed from that at the heating sources because of the existence of the time for heat transfer from the heating sources to the surface. As a further problem, the heat capacity of the base plate 21 is so large that the temperature of the base plate surface will not fall rapidly after the electric current is switched off.
FIG. 10 illustrates another method by which flat-type electric heaters 26 with nichrome wires housed in a flat-type metal container are fitted to a base plate 21 to achieve the same purpose as the method mentioned above. By this method, although non-uniformities of the surface temperature can be reduced to a smaller extent than by the aforementioned method, the delay of the heating time and cooling time cannot be avoided, also because of the large heat capacity of the base plate 21.
To improve these and other drawbacks, there has been proposed a method, as shown in FIG. 11, in which a thin film electric resistor (thin film electric heater) 10 is formed on a surface of an electrically insulating object 21 and the object surface is heated by electric current supplied to the thin film electric heater 10 through a conductor wire 14 and terminals 13, 18. This type of proposal is made by, for example, JP-A 49-53953 and JP-A 55-67417. However, even with the use of these proposals, effective heating action cannot actually be attained without satisfying further requirements for the value of input power, the value of electric resistance, and the like. Nevertheless, the aforementioned prior art arrangements include no specific requirements with regard to these factors.
Particularly, in order to get a rapid heating of the object surface, more restrictive requirements are needed. However, the above prior art arrangements do not describe at all such restrictive requirements.
The method of heating by using a thin film electric heater formed directly on a surface of an electrically insulating object is an effective method for heating the surface of the object, particularly for heating a surface of an object rapidly. However, satisfactory heating effects cannot be obtained only by forming a thin film electric heater directly on the object surface. Other requirements must be satisfied, to achieve rapid heating.
It is a first requirement for heating an object surface that the quantity of heat supplied to the object surface should be greater than that dissipated from the object surface. Namely, the first requirement is that the quantity of heat produced by the thin film electric heater formed on the surface should be larger than that dissipated from the object surface per unit time.
Next, in order to satisfy the first requirement, limitations on voltage and current must be observed due to the capacity of the power source and safety. When the operating voltage is limited to be low, in order to meet the first requirement, the large required current should be obtained by using a lower resistance value of thin film electric heater. But, in some cases, such a large electric current may exceed the allowable current. Conversely, under the limitation of low operating current, the supplied voltage should be high enough to meet the first requirement by using a higher resistance value of the thin film electric heater. But, in some cases, such a high voltage may exceed the allowable voltage. Accordingly, in addition to meet the first requirement, the values of the voltage and the current must be in allowable ranges by using a adequate resistance value. This is a second requirement.
Generally, the resistance value of the thin film electric heater depends on the intrinsic volume resistivity of the material used and the dimensions of the thin film electric heater. In order to meet the first and second requirements, these are some cases where a thin film electric heater with a thickness under 1 .mu.m has to be used, depending on the intrinsic volume resistivity of the material, which may result in poor working reliability. In other converse cases where a thin film electric heater with a large thickness must be used due to the intrinsic volume resistivity of the material, such excessive large thickness is not practical. As a consequence, proper material selection and optimum design for the thin film electric heater are required. This is a third requirement.
However, there have been no thin film electric heaters, so far, that satisfy such three requirements.
The process of injection molding of plastics includes injecting, cooling, and solidifying molten plastics in a mold which has a cavity whose geometrical shape is identical with that of a desired product.
This injection molding is conducted by apparently simple processes, but there are various problems in actual practice. For example, there may occur problems of "weld-mark", "silver streak" and "sink mark" upon the appearance of products. Herein, the term "weld-mark" refers to a phenomenon that streaky flaws appear on the product surface, and this phenomenon often occurs around openings of the product. The structures at the opening portions of a mold hinder the filling of plastics. When the flow front of molten plastics flowing within the cavity reaches the opening portions, the flow of the molten plastics is branched around the opening portions and meets again to flow together. During a quite short time interval from branching to meeting, the surface of the plastics comes into contact with the mold wall surface and air, thereby being cooled, so that an extremely thin frozen layer is formed. As a result, at the time when the branched flows meet via the frozen layer, such a frozen layer is apparent as a streaky flaw. The term "silver streak" refers to surface defects appearing as if the surface were scratched by a brush. This phenomenon occurs because moisture contained in the plastics vaporizes into steam and, in some cases, the plastics itself decomposes to generate gases, so that minute foams due to these gases gather and appear on the product surface. Furthermore, the term "sink mark" refers to surface defects due to small depressions on the product surface. This phenomenon occurs, in many cases, at places where the wall thickness of the product changes abruptly.
In actual injection molding operations, various efforts such as adjustment of the temperature of molten plastics, injection pressure, injection speed, or modification of the mold structures have been attempted to reduce such defects. However, in fact, it is quite difficult to eliminate such defects completely.
As an effective method for eliminating such defects, there is conventionally known a method in which the mold is heated. By heating the mold, since the growth of the frozen layer on the surface of fluidized plastics can be suppressed, the branched flows of plastics will meet without traces. As a result, the "weld-mark" defect can be eliminated. Further, a higher mold temperature will lower the viscosity of the plastics low. Thus, even if any foam exists it will be crushed by the injection pressure, so that "silver streak" defect also will be eliminated. However, it has not necessarily been possible to improve the "sink mark" defect by increasing the mold temperature. Still, the aforementioned method has been advantageous for improvement of the "weld-mark" and "silver streak" problems.
As described above, it has been found that plastics injection is improved by heating the mold. However, heating of the mold prolongs the molding time, which in turn results in higher cost of the injection molding operation.
As typical conventional methods of heating the mold surface, electric heaters, for example, as shown in FIGS. 9 and 10, are incorporated into the mold to perform heating. By this method, it takes a long time to heat the mold to a predetermined temperature, and further to cool the plastics injected into the mold. The quantity of heat for raising the temperature of the wall surface of the mold is only a small portion of the entire quantity of heat generated by the electric heaters, because the generated heat diffuses into the whole mold. Accordingly, a long time is required to raise the temperature of the wall surface of the mold. Conversely, it is required to remove the entire quantity of heat accumulated in the whole mold in order to cool the wall surface. This causes the problem that longer cooling time also is needed.
This fact is incompatible with the demand of molding manufactures that the molding time be reduced as much as possible. Therefore, these conventional methods of using electric heaters to raise the mold temperature are not acceptable for practical use.
In another proposed method, instead of using the electric heater, the mold may be heated by flowing high temperature fluid through cooling channels originally incorporated for cooling the mold. However, the disadvantages are the same as for the foregoing method.
Further proposed is a method in which only the mold surface is heated with high-frequency current. Since the high-frequency current will flow only through the surface of conductors, this method is suitable for the heating of the mold surface alone. Therefore, this method has an advantage that the time needed for heating is shortened. However, in order to generate a high-frequency current on the mold surface, it is necessary to place a high-frequency coil opposed to the mold surface to be heated. Therefore, during the time when movable and stationary molds are kept open for ejecting the molded product, the high-frequency coil is between the molds. This would result in a prolongation of the overall molding time. Yet, after the mold surface is heated to a predetermined temperature with a high-frequency current, the temperature would fall during the time of moving the high-frequency coil out of the molds.
As mentioned above, the conventional methods of heating the mold have the disadvantage of prolonging molding time.
The present invention is therefore intended to provide a thin film electric heater which makes it possible to heat only the surface of an object rapidly, and to provide a method and apparatus or mold for plastics molding which use the thin film electric heater in a mold for injection molding, so that the prolongation of molding time is reduced to minimum, and that the defects such as "weld-mark", "silver streak" and "sink mark" rarely occur.